Women diagnosed with gestational diabetes mellitus (GDM) are at substantially increased risk of developing type 2 diabetes and obesity, currently at epidemic rates in the United States. GDM, therefore, identifies a population of women at high risk of developing type 2 diabetes and thus provides an excellent opportunity to intervene years before the development of this disorder. It is well recognized that acute as well as chronic physical activity reduce fasting plasma glucose as well as improve glucose tolerance in type 2 diabetes. Recent epidemiologic studies have suggested that women with higher levels of physical activity have reduced risk of GDM. Therefore, we propose to test the hypothesis that an exercise intervention is an effective tool for preventing GDM among women with a history of GDM. The primary goals of the application are to investigate the effects of a motivationally-tailored, individually targeted 12-wk physical activity intervention on 1) risk of recurrent GDM, 2) serum biomarkers associated with insulin resistance, 3) and the adoption and maintenance of exercise during pregnancy. Secondary goals are to investigate the impact of the intervention on gestational weight gain and selected birth outcomes. A total of 364 multiparous women who had GDM in a prior pregnancy (49 percent will be from minority groups) will be recruited in early pregnancy (10 wks gestation) and randomized to either an Individually Targeted (IT) intervention (n=182) or a Comparison Health and Wellness (HW) intervention (n=182). The overall goal of the intervention is to encourage pregnant women to achieve the American College of Obstetricians and Gynecologists (ACOG) Guidelines for physical activity during pregnancy (30 minutes or more of moderate- intensity activity on most days of the week) through increasing walking and developing a more active lifestyle. The intervention draws from the theory of Stages of Motivational Readiness for Change and Social Cognitive Theory constructs for physical activity behavior and will take into account the specific social, cultural, economic, and physical environmental challenges faced by women of diverse socioeconomic and ethnic backgrounds. The application is innovative in being the first, to our knowledge, to test a physical activity intervention designed to prevent GDM among high risk women. The intervention protocol can readily be translated into clinical practice in underserved and minority populations.